1. Field of the Invention
The present invention relates to an arrayed waveguide grating optical multiplexer/demultiplexer and an expanding width waveguide.
2. Discussion of the Background
In recent optical communications, research and development of optical wavelength division multiplexing communications has actively been made as a way to exponentially increase the transmission volume, and the results are being put into practice. The optical wavelength division multiplexing communications uses, for example, a technique of wavelength division multiplexing on a plurality of light beams each having a wavelength different from one another to transmit them. In the system of such optical wavelength division multiplexing communications, an optical multiplexer/demultiplexer is necessary which multiplexes a plurality of light beams each having a wavelength different from one another and which demultiplexes light that has undergone wavelength division multiplexing to be transmitted to create a plurality of light beams each having a wavelength different from one another.
FIG. 18(a) illustrates an arrayed waveguide grating (AWG) type optical multiplexer/demultiplexer. Referring to FIG. 18(a), the arrayed waveguide grating type optical multiplexer/demultiplexer is obtained by forming on a substrate 11 an optical waveguide unit 10 that has a waveguide structure.
The waveguide structure includes at least one optical input waveguide 12 arranged side by side, a first slab waveguide 13 connected to the exit ends of the optical input waveguides 12, an arrayed waveguide 14 connected to the exit end of the first slab waveguide 13, a second slab waveguide 15 connected to the exit end of the arrayed waveguide 14, and a plurality of optical output waveguides 16 that are arranged side by side and connected to the exit end of the second slab waveguide 15.
The arrayed waveguide 14 propagates light that is outputted from the first slab waveguide 13, and is composed of a plurality of channel waveguides (14a) that are arranged side by side. Lengths of adjacent channel waveguides (14a) are different from each other by a predetermined length difference (AL). The optical input waveguides 12 and the optical output waveguides 16 have the same dimension.
The number of optical output waveguides 16 is determined, for example, in accordance with the number of light beams which have different wavelengths and which are created by demultiplexing signal light with the arrayed waveguide grating type optical multiplexer/demultiplexer. The arrayed waveguide 14 usually includes a large number (for example, 100) of the channel waveguides (14a). However, FIG. 18(a) is simplified and the number of the channel waveguides (14a), the optical output waveguides 16, and the optical input waveguides 12 in FIG. 18(a) does not exactly reflect the actual number thereof.
FIG. 18(b) schematically shows an enlarged view of an area of FIG. 18(a) which is surrounded by a dotted line (A). As shown in FIG. 18(b), in the arrayed waveguide grating type optical multiplexer/demultiplexer in the background art, the substantially straight portion (12a) which is connected to an end portion of the slightly curved portion (12b) of the optical input waveguides 12 is directly connected to the entrance end of the first slab waveguide 13. Similarly, the substantially straight portion which is connected to an end portion of the slightly curved portion of the optical output waveguides 16 is directly connected to the exit end of the second slab waveguide 15.
One of the optical input waveguides 12 is connected to, for example, transmission side optical fiber, so that light having undergone the wavelength division multiplexing is introduced to one of the optical input waveguides 12. The light which has traveled through one of the optical input waveguides 12 and been introduced to the first slab waveguide 13 is diffracted by the diffraction effect thereof and enters the arrayed waveguide 14 to travel along the arrayed waveguide 14.
Having traveled through the arrayed waveguide 14, the light reaches the second slab waveguide 15 and then is condensed in the optical output waveguides 16 to be outputted therefrom. Because of the preset difference in length between adjacent channel waveguides (14a) of the arrayed waveguide 14, light beams after traveling through the arrayed waveguide 14 have phases different from one another. The wavefront of the light beam is tilted in accordance with this difference and each position where each light beam is condensed is determined by the angle of this tilt. Therefore, the light beams having different wavelengths are condensed at positions different from one another. By forming the optical output waveguides 16 at these positions, the light beams having different wavelengths can be outputted from their respective optical output waveguides 16 that are provided for the respective wavelengths.
For instance, as shown in FIG. 18(a), the light having undergone the wavelength division multiplexing and having wavelengths of λ1, λ2, λ3, . . . , λn (n is an integer equal to or larger than 2), is inputted from one of the optical input waveguides 12. The light is diffracted in the first slab waveguide 13, reach the arrayed waveguide 14, and travel through the arrayed waveguide 14 and the second slab waveguide 15. Then, as described above, the light beams are respectively condensed at different positions determined by their wavelengths, enter different optical output waveguides 16, travel along their respective optical output waveguides 16, and are outputted from the exit ends of the respective optical output waveguides 16. The light beams having different wavelengths are taken out through optical fibers that are connected to the exit ends of the optical output waveguides 16.
In this arrayed waveguide grating type optical multiplexer/demultiplexer, improvement in wavelength resolution is in proportion to the difference in length (AL) between the channel waveguides (14a) of the arrayed waveguide 14. When the optical multiplexer/demultiplexer is designed to have a large (ΔL), it is possible to multiplex/demultiplex light to accomplish wavelength division multiplexing with small wavelength differences, which has not been attained by any conventional optical multiplexer/demultiplexer. It is thus possible for the optical multiplexer/demultiplexer to have a function of multiplexing/demultiplexing a plurality of signal light beams, specifically a function of demultiplexing or multiplexing a plurality of optical signals with a wavelength difference of at most 1 nm. High density optical wavelength division multiplexing communications require such a small wavelength difference.
The arrayed waveguide grating type optical multiplexer/demultiplexer is obtained by, for example, forming a waveguide formation region 10 having the above waveguide structure on a substrate 11 made of silicon (Si) as follows:
That is, an under cladding layer (SiO2 based glass) and a core layer (for example, glass mainly containing SiO2 to which GeO2 is added) are formed on the substrate 11 by flame hydrolysis deposition method, and the above waveguide structure is formed by, for example, photolithography and reactive ion etching method. Subsequently, the over cladding layer that covers the waveguide structure of the core is formed by flame hydrolysis deposition method.
Japanese Unexamined Patent Publication (Kokai) No. Hei 5-313029 discloses an arrayed waveguide grating type multiplexer/demultiplexer. The contents of this reference are incorporated herein by reference in their entirety. In this multiplexer/demultiplexer, optical input waveguides are connected to an inputside slab waveguide via a tapered waveguide.
Japanese Unexamined Patent Publication (Kokai) No. Hei 8-122557 discloses an arrayed waveguide grating type multiplexer/demultiplexer. The contents of this reference are incorporated herein by reference in their entirety. In this multiplexer/demultiplexer, an optical input waveguide is connected to an inputside slab waveguide via a tapered waveguide which has a slit along a center axis of the tapered waveguide.
Japanese Unexamined Patent Publication (Kokai) No. Hei 9-297228 discloses an arrayed waveguide grating. The contents of this reference are incorporated herein by reference in their entirety. In this arrayed waveguide grating, optical input waveguides are connected to an inputside slab waveguide via a parabolic waveguide.